A number of different types of can constructions have been developed for packaging food products. For instance, soldered or welded “three piece cans” are well known in which individual can body blanks are fed into a body maker where the cylinder is formed, seamed, and flanged, and then the bottom end is separately applied before filling the can, and the top end thereafter. “Two piece cans” are manufactured from thin sheets of aluminum or steel in which the can body and bottom end are integrally formed.
Steel has the advantage of being magnetic which facilitates recycling. Steel sheets for food containers are usually coated with a metal coat (tin or chrome), on which there is generally deposited an organic barrier coating. Two-piece steel containers are made by deep-drawing a steel blank under a blank holder in one (single draw) or more (draw-redraw) operations. The resulting open can structure has a cylindrical body and integral bottom end (maker's end), while the opposite end is open at this juncture. Thicknesses of steel sheets used for such can structures generally have ranged from about 0.08 mm to about 0.25 mm, although greater or smaller thicknesses have been used for particular applications. As the top end (customer's end) of the can, which is separately attached after filling the open can structure with food, a number of different can ends have been used, including round ends, non-round ends, pull-tab can ends, key-open ends, and foil laminated tinplate lids.
Pull-tab can ends for two-piece food cans are widely used. They are made from flat profile ends constructed of aluminum or steel. The ends are fed into a conversion press in which the end is scored, the flat profile modified with strengthening and convenience features and the rivet is formed. Tab stock is fed into the press where the pull tab is formed. The pull tab then advances to the modified basic end to which it is attached at the rivet. A pull-tab can end is seamed onto a can after it has been filled with food product with a closing machine. Closing machines are variously equipped to apply an end to a can after filling under a number of specific conditions dependent on the food product and the packer's needs such as vacuum closure, steam closure and vacuum gas closure. “Easy open ends” are a popular type of pull-tab can end allowing substantially complete removal of a panel covering an end of the container without the need to use a can opener or similar tool.
Many food products are hot-filled in two-piece container systems. Release problems have been experienced with two-piece metal cans hot-filled with certain food products. Food products, such as process cheeses, cheese spreads, and the like, can be conveniently filled in a hot molten state into the can. However, upon cooling and solidifying, these types of food products often tend to stick to and/or become “gripped” by the inner container wall. As a consequence, product users may need to use a utensil, such as a spoon, to tediously scrape off, scoop off, or otherwise manually separate and dislodge cheese portions from the inner container wall. As generally known in the packaging arts, when the hot-filled food contents of a closed container cool, they tend to shrink in volume, causing an internal partial vacuum effect in the container. Condensation of moisture in headspace in the container can intensify the vacuum effect. The vacuum effect tends to create an inward pulling force on the container walls. Depending on the structural rigidity of the container wall, inward deformation or a slight collapsing of the container wall can occur due to the vacuum effect sufficient to cause the container wall to press upon and “grip” the food contents. Thin metal container walls in particular, once deformed in this manner, tend to stay deformed even after the food container is ultimately opened. Ideally, the food product would readily release from the inner container wall so that it can be served or dispensed more easily. The use of thicker and thus structurally more rigid metal container wall materials may reduce adverse consequences of vacuum effect, but has disadvantages of increasing packaging costs and possibly creating container forming problems.